Solid-state imaging devices such as a CMOS sensor are decreasing their pixel size year after year to meet demands for more pixels and smaller optical sizes. For example, the pixel size of a CMOS sensor used in a digital camera or the like has reached about 2 μm to 3 μm recently.
A smaller pixel size results in a smaller area of a photodiode arranged in a pixel. Hence, the number of saturated electrons accumulated in the photodiode decreases, failing to ensure sufficient signal charges. Shortage of signal charges decreases the S/N ratio, and noise stands out on the reproduction screen.
For example, to increase the number of saturated electrons in the photodiode, there is proposed a layout in which four pixels share one floating diffusion to decrease the number of floating diffusions and increase the photodiode area.
In the layout of four pixels in one cell in which four pixels form one unit cell, the floating diffusion is arranged at the center and surrounded by four pixels. This layout can reduce the number of floating diffusions, compared to, e.g., a layout of two pixels in one cell in which two pixels share one floating diffusion. This can increase the photodiode area and thus increase the number of saturated electrons accumulated in the photodiode.
However, in the layout of four pixels in one cell, when reading out signal charges from pixels arrayed on one row, while signal charges are read out from one pixel, no signal charge can be read out from its adjacent pixel, failing speedup of the readout operation.